1. Field of the Invention:
This invention relates to a material for a microwave dielectric resonator.
2. Description of the Prior Art:
The development of new media for communications has necessitated the use of small, digital electronic parts and devices. There is a particularly strong demand for small and reliable parts for communication systems employing microwaves, such as communication satellites and automobile telephones. Conventional microwave circuits employ cavity resonators and waveguides as filters and transmission lines, which essentially rely on the stability of air or vacuum as a wave propagation medium. In order to reduce the sizes of these parts, it has been found possible to use a propagation medium having a higher permittivity and a higher temperature stability as compared to that of air or vacuum. In this regard, the propagation wavelength of microwaves in a medium is expressed as ##EQU1## .epsilon..sub.r relative permittivity) and its resonance wavelength is also expressed as ##EQU2## It is, therefore, possible to reduce the sizes of those parts.
Various types of ceramics are known for making microwave dielectric resonators. They include CaZrO.sub.3, SrZrO.sub.3, CaTiO.sub.3 --MgTiO.sub.3,Ba.sub.2 Ti.sub.9 O.sub.20 and BaTi.sub.4 O.sub.9, e.g., as disclosed in the article "Ba.sub.2 Ti.sub.9 O.sub.20 as a Microwave Dielectric Resonator" in the Journal of the American Ceramic Society, 1975, Vol. 58, No. 9-10, pp. 418-420 and in the article "Dielectric Characterization of Ba.sub.2 Ti.sub.9 O.sub.20 type Ceramics at Microwave Frequencies" in the Philip Journal of Research, 1983, Vol. 38, No. 6, pp. 295-311. The first three types of ceramics have a relatively widely ranging temperature coefficient of permittivity including zero, but a low relative permittivity of only, for example, 20. The last two types have a relatively high relative permittivity of, for example, 40, but their temperature coefficient of permittivity is not as low as zero--see U.S. Pat. No. 3,938,064 and the above-mentioned article in the Journal of the American Ceramic Society. Also, they are liable to reduction and difficult to sinter as disclosed in the article "Effect of Mn Doping on the Dielectric Properties of Ba.sub.2 Ti.sub.9 O.sub.20 Ceramics at Microwave Frequency" in Japanese Journal of Applied Physics, 1983, Vol. 22, No. 6, pp. 1125-1128.
In the event a dielectric resonator is used as a filter, the shape of the filter and the expansion of metals, such as of a metal casing for the filter and cables, have a noticeable effect on the temperature coefficient of resonance frequency of the filter as a whole. Those metals make a positive contribution to the temperature coefficient of resonance frequency.
There have also been proposed BaO--SnO.sub.2 --TiO.sub.2 and BaO--ZrO.sub.2 --TiO.sub.2 types of ceramics. They are, however, both limited in temperature coefficient of permittivity. Their temperature coefficients of resonance frequency are only as low as about zero. It is difficult to obtain any such type of ceramics having a negative temperature coefficient of resonance frequency. In order to lower the temperature coefficient of the filter as a whole, therefore, it is necessary to use for the dielectric resonator itself a material having a negative temperature coefficient which makes up for that of those metals.